In recent years, the editing of image, graphic or the like by a computer is conducted in various fields. For example, in a Geographic Information System (GIS) and the like, a graphic such as a polyline and a polygon is represented by a vector data. In order to move a linear graphic represented by a vector data, a user clicks the mouse on a vertex of the graphic and drags the mouse to move the graphic. On the other hand, in order to edit a vector data, a user adds a new vertex to a side of the graphic and moves the vertex. If a user specifies a vertex and opens a pop-up menu or the like to select a command to eliminate the vertex, the unnecessary vertex can be eliminated.
As another method for editing a graphic, there has been disclosed in Japanese Published Unexamined Patent Application No. 5-108786 a method in which a line segment to be edited on a screen is made to overlap with a polygon for editing the line segment, thereby transforming that part of the line segment which overlaps the polygon into the shape of the periphery of the polygon.